In a semiconductor integrated circuit device required to be operated at a high speed, in accordance with recent refinement of semiconductor devices, increase in the sheet resistance and the contact resistance of a semiconductor layer including an impurity dispersed therein has become a serious problem.
As one means for overcoming this problem, a process for forming a silicide layer in a surface portion of the semiconductor layer has been proposed. A variety of metals have been proposed to be used for forming the silicide layer, among which a cobalt disilicide (CoSi2) layer formed by using cobalt is regarded as a particularly promising silicide layer because it is good in both thermal stability and specific resistance.
In the case where a surface portion of a silicon substrate is silicided by using cobalt, however, agglomeration can be caused in the cobalt silicide layer or a spike defect can be caused in the cobalt silicide layer during a reaction process occurring between cobalt atoms and silicon atoms (IEDM 1995-449, K. Goto). The agglomeration of the cobalt silicide layer leads to open circuit failure, and the occurrence of a spike defect leads to junction leakage.
As means for preventing the agglomeration and the occurrence of a spike defect in the cobalt silicide layer, a method for forming a cobalt silicide layer by epitaxial growth as follows is proposed in an article (Appl. Phys. Lett. 68, 1996, June). Specifically, the article proposes a technique of epitaxial growth of a cobalt silicide layer (oxide mediated epitaxy; OME) by forming a SiOx (x<2) film with a thickness of 0.5 through 1.5 nm on a semiconductor layer of silicon crystal, depositing a cobalt film in approximately several nm on the SiOx film under ultra high vacuum and carrying out annealing for causing a reaction between cobalt atoms and silicon atoms. It is also described that the SiOx film plays a role for promoting the growth of the cobalt silicide layer.
However, the aforementioned method for forming a cobalt silicide layer by the epitaxial growth requires ultra-high vacuum equipment for the deposition of a cobalt film, and the ultra-high vacuum equipment is not employed in a general process for semiconductors of silicon. Therefore, this method is not suitable to the mass production process.
Furthermore, since the cobalt film is formed on the SiOx (x<2) film having a very small thickness and including silicon excessively as compared with a stoichiometric composition, various problems occur in the aforementioned method due to variation in the film quality and the thickness of the SiOx film. Specifically, in the case where a pin hole is formed in the SiOx film, cobalt and silicon are explosively reacted to each other through the pin hole, and hence, there arises a problem that the cobalt silicide layer cannot be epitaxially grown; and in the case where the thickness of the SiOx film is fluctuated, the reaction between the cobalt atoms and the silicon atoms is too rapidly proceeded in a portion with a small thickness, and hence, there arises a problem that the cobalt silicide layer cannot be satisfactorily epitaxially grown.